I am working on a design that uses the I210IT part. We are the OEM, there is no third party. To provide the required 25Mhz clock signal at the XTAL1 pin of the I210 part, the data sheet recommends using a crystal but then shows information on using an oscillator. In section 126.96.36.199 ( External Clock Oscillator Specs), the data sheet shows an oscillator powered by 2.5V and 3.3V feeding a capacitor divider circuit whose purpose is to get the voltage to the right level before it goes into the XTAL1 pin. Specs are listed in Table 11-16 for the XTAL1 pin indicating it is a 1.8 volt input. What is the purpose of using an oscillator powered by 2.5 or 3.3 volts? Is there a problem with providing a clock signal with 1.8V logic levels per Table 11-16? Note that there are oscillators that run off of 1.8 volts.
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It is important to let you know that the information stated in the documentation is the proposed and corroborated by Intel.
Due to this fact, you can implement your suggestion but it should be tested and validated on your own.